1. Field of the Invention
The field of the invention relates to semiconductor circuits and in particular, to voltage level shifters for switching between a first value and a boosted second value.
2. Description of the Prior Art
Circuits that can switch from a first voltage level to a boosted second voltage level are known. For example in the field of power gating circuits to reduce power consumption during periods of no activity, it is known to provide header or footer devices that gate the power supply to processing circuitry. These devices take the form of transistors that lie between one of the power rails and the processing circuitry and are turned on or off by a control signal applied to their gates. When they are off, there is still some leakage current that flows through the transistor and then through the processing circuitry and this leakage current increases the power consumption of the device without providing any benefit. This problem has been recognised and one solution has been to provide a gate bias voltage that biases the voltage applied to the gate beyond the supply voltage resulting in a state that is sometimes referred to as a super cutoff state. In this state the leakage current through the transistor is significantly reduced.
A problem with such a system is that an additional power supply is required to supply this bias voltage. This power supply can be provided by an on chip voltage regulator to generate a boosted core supply or by a charge pump. In order to reduce the size requirements of such devices it is desirable to keep the current rating of these devices small. A low current also reduces the grid requirements associated with the supply, that is the size of the connections required to distribute current from the supply to the required nodes. However, restricting the amount of current that such devices can supply can affect the performance of the system by slowing down the time taken for some transitions.
It would be desirable to be able to provide a boosted voltage level without a large additional hardware requirement.